1. The problem statement, all variables and given/known data A pi zero meson is an unstable particle produced in high energy particle collisions. It has a mass-energy equivalent of about 135MeV, and it exists for an average life-time of only 8.7x10^-17 seconds before decaying into two gamma rays. Using the uncertainty principle, estimate the fractional uncertainty Δm/m in its mass determination. 3. The attempt at a solution Ok, I know mc^2 = 135 MeV = 2.16x10^-11 J Δt = 8.7x10-17 s And I suspect I am supposed to use ΔtΔE ≥ h/(4pi) Taking a wild stab in the dark here based off of some examples I've looked at: ΔE/E = Δm/m ? if so, I am given E, and the time-energy uncertainty equation will give me ΔE, so I can calculate Δm/m only I'm getting the wrong answer. This question is addressed in a previous thread, located here, where Javier suggests using ΔEΔt = h/2pi instead of h/4pi When I use this method to calculate ΔE, then use ΔE/E = Δm/m, I get the correct answer. Why do I have to use h/2pi instead of h/4pi?